This project is directed at a more complete understanding of fatigue failure as it occurs in bone and at the relationships that different exercise regimens have on the mechanical and geometric parameters of bone and the biological reaction of bone to these exercise programs. Since fatigue fractures and fatigue injury occur so frequently it is the hope of the investigators that individual exercise programs could be started to prevent and/or treat fatigue injuries of bone. The authors propose to use the horse as a spontaneous model of fatigue failure and to test the hypothesis that high strain exercise caused decreased stiffness in bone in vivo much as it does in vitro. The biological reaction to this high strain in young individuals is directed at changing the inertial properties of the bone to allow bone remodeling to reconstitute the bone's architecture. The authors propose to reconstruct the fracture injury in bone using image analyses techniques to allow for 2-plane reconstructions. Measurement of in vivo bone strain will be accomplished before and after fatigue failure. Bone modeling and remodeling will be examined using bone labels and calcified histology. Changes in bone stiffness will be monitored in vivo using strain gage instrumentation. In vitro measurements using 4-point bending tests will be made and compared to predicted measurements using data from computed tomography scans of material and inertial properties in normal bones and in those subjected to high strain cyclic fatigue.